The present invention relates to a mechanical shaft joint boot which is made of polymeric elastomer such as rubber and provided with bellows between a large-diameter ring and a small-diameter ring, the first crest of the bellows (numbered from the large-diameter side, the troughs being serially numbered from the same side hereinafter) being formed between the large-diameter and small-diameter rings after the first trough. More particularly, the invention relates to a boot suitable for a mechanical shaft joint of a constant velocity-type which provides a joint between a driving shaft and an axle of a passenger car.
Such a constant velocity-type mechanical shaft joint boot 31 (referred to as boot hereinafter) has heretofore had such a structure that the first crest 39 of the bellows 36 between the large-diameter ring 33 and the small-diameter ring 35 has been directly connected to the large-diameter ring 33, as shown in FIG. 1. However, there is a tendency for engine compartments to be made small nowadays as motor cars are often designed with front engine front drive (thereby providing an increase in the size of the passenger compartment). It has therefore become desirable for a joint part and a boot to be of such a design that the boot has a small circumferential diameter, i.e. the space occupied by the first crest 39 (containing a whirling property) must be minimized. A boot 21, as shown in FIG. 2, is being placed on the market, in which a first trough 28 is interposed between the large-diameter ring 23 and the first crest 29 of the bellows 26 provided between large-diameter and small-diameter rings 23 and 25. In the figures, reference numeral 11 denotes a driving shaft and reference numeral 12 denotes a shaft joint.
However, if the circumferential diameter of the first trough is made small so as to further restrain the whirling property, the boot of the above-described type (the boot shown in FIG. 2) has suffered the following problems which need to be overcome: (a) It might first be considered that making the circumferential diameter of the first trough 28 smaller (the trough thus being deepened) would attain the purpose of restraining the whirling property of the first crest 29 which represents the maximum circumferential diameter of the boot. However, by deepening the first trough, the first trough 28 was made susceptible to being broken when disposed between a socket 13 and driving shaft 11 where a large angle is formed between the axis of the driving shaft 11 and the axis of the socket 13 (hereinafter referred to as "crossing angle"), reduction in the circumferential diameter of the first trough 28 thus being limited. (b) Next, it might be considered feasible to decrease the circumferential diameters of others of the troughs, i.e. the ones on the small-diameter ring side of the second trough 27, but if the driving shaft 11 and the socket 13 made a crossing movement with a crossing angle of 30.degree. (FIG. 3), the second trough 27 would then be brought into a state of being held between the first trough 28 with the increased circumferential diameter and the driving shaft 11, and the distortion would then be particularly concentrated at the region A in the figure so that the bending fatigue of the second trough 27 would thereby be accelerated leading to a reduction in the durability of the boot 21.